The present invention relates to a laser diode driver, and more particularly to the bias current control of the output to reduce the overall power consumption of the driver, interconnects and laser diode (sometimes referred to as “diode laser”).
It is well known in the laser diode arts that the drive current applied to a laser diode must exceed a certain threshold current level before the laser diode will emit laser light; i.e., the laser diode turns on. If the amount of current that is driving the laser diode is below this threshold level, then the laser diode remains dark.
In order to turn on laser diodes quickly, the diodes are normally driven by a bias current. Conventionally operated laser diode drivers provide a bias current that is applied to (i.e., drives) the laser diode at a non-zero current level that is insufficient to turn the laser on (i.e., less than the turn-on threshold current). Conventionally, this bias current is a constant DC level and can be anywhere from a few milliamps to several Amps. When a main output current (the drive current) is applied to the laser diode, the laser diode can turn on much more quickly than if the laser diode was not being driven by a bias current. The laser diode can be made to turn on nearly instantly by properly setting the DC level of the bias current close to the turn-on threshold current level. It is common practice for the user to find a suitable DC level for the bias current by straightforward trial and error, since laser diode characteristics, such as the threshold level, typically vary from manufacturer to manufacturer.
The turn-on threshold current level is known in the laser diode industry as “the bias current” or “the simmer current.” However, in order to distinguish the actual current that is applied to the laser diode versus the current level above which the laser diode turns on, the latter will be referred to herein variously as “turn-on current level,” “turn-on threshold current level,” and so on. The actual current that is applied to the laser diode will be referred to as the bias current.
The typical operation of a laser diode driver instrument is shown by the graph in FIG. 13. The x-axis is time (milliseconds). The left-side y-axis is current (Amps) and the right-side y-axis is light output (Watts). FIG. 13 shows a trace for the output current of the instrument consisting of a bias current level and the drive current. FIG. 13 shows the instrument is configured to output a constant current of 25 Amps (the bias current) and to output a current of 95 Amps (the drive current). The light output of the laser diode (expressed in units of Watts) is shown as a trace that is superimposed on the trace of the output current of the instrument.
When the output current of the instrument consists only of the bias current (e.g., at the time between 0 and 20 mSec), the laser diode emits no light, even though it is being driven by the bias current. When the drive current is added to the bias current, then the laser diode emits light; i.e., it is turned on. Moreover, by virtue of driving the laser diode with a bias current prior to applying the drive current, the laser light turns on almost immediately as shown by the steep (step function-like) profile of the light output trace. If the laser diode is not pre-driven by a bias current, the light output trace of the laser diode would have more of a triangular profile (for example, see the inset in FIG. 13), as the laser slowly turns on and off Many applications require a sharp, instant-on type of laser burst. For example, when doing distance measurements, the user wants a bright laser burst and then for the laser to turn off. For such applications, the use of a bias current is an important part of laser diode operation.
Lower bias current levels do not use very much power, but the light profile that results by operating the laser diode with a lower bias current may not be adequate. However, as the bias current level is increased to a suitable level, the laser diode driver instrument and overall laser system will consume more power. Accordingly, a user will adjust the bias current level to as low a level as possible while still achieving a suitable immediate turn-on behavior from the laser diode.